In many process applications, the dew point of air in a particular environment must be closely controlled below a predetermined level. For example, in pneumatically operated fluidic systems, particularly fluidic control systems, if the dew point of the pneumatic air is reached or exceeded, the condensed water can adversely affect the finely tuned pneumatic elements. In a radio frequency environment where RF energy is being pumped through a coaxial cable or a waveguide, water vapor can cause arcing which dissipates energy and causes degradation of the circuits.
Of the various systems available in the prior art to control the dew point of a gas, particularly air, below a certain level, one such system is a membrane gas dehydrator. The invention disclosed herein is an improved membrane gas dehydrator.
FIG. 1 is a schematic of a prior art membrane gas dehydrator shown generally at 10 such as would be used for removing vapor from a compressed air stream for a fluidic control system. Compressed air containing water vapor enters an inlet plenum 12 at an elevated pressure, P1. The moisture laden air then flows through the interior of a plurality of membrane capillary fibers 14. The fibers are constructed of a material with microscopic porosity. The size of the pores is carefully controlled to allow the passage of water molecules but to retard the passage of gas molecules for example, nitrogen and oxygen molecules, the major constituents of air.
At the far end of the fibers 14, water depleted gas enters an exit plenum 16 and then to the user's gas distribution system via conduit 18. The water molecules are driven through the membrane walls of the tubes 14 by the difference in water vapor pressure between the inside and outside of the fibers. The water vapor coalesces on the outside surface of the fibers. The water vapor which coalesces equalizes the internal and external water vapor pressure thereby slowing or stopping the drying process.
In order to remove the permeated water vapor from the external surface of the fibers 14 it is conventional to provide a sweep or purge flow of dry air from the outlet of the dehydrator outlet conduit 18. This basically is a bypass 20 which leads to a throttle valve 22 such as a "HOKE", Inc., Series 1600. The metered gas then flows through a conduit 24 to an inlet in the shell 26. The metered gas sweeps or purges the collected water vapor from the external surfaces of the fibers and exhausts it to the atmosphere via a vent port 28.
At a given operating pressure and inlet dew point temperature (water vapor content) the exit dew point of the gas will vary inversely with the purge flow rate. Thus, the user can adjust the purge flow rate until the user achieves the desired outlet dew point at the desired output flow rate. However, if the operating pressure is increased while the inlet dew point is held constant, the water vapor pressure is increased. Because the water vapor pressure is the force which causes water vapor to permeate through the fibers more water will permeate resulting in a lower exit dew point for a given exit flow rate and purge flow rate. However, because the purge flow rate is established by a fixed orifice, if the operating pressure is increased, the purge flow rate will likewise increase proportionately to the pressure increase. Because the desired exit moisture content had already been achieved at the lower purge flow rate set for the lower operating pressure this additional purge flow is wasted and represents a decrease in the efficiency of the dehydrator.
In the present invention, a pressure regulator is incorporated into the prior art bypass stream. The regulator is set to deliver a pressure somewhat lower than the lowest anticipated operating pressure and the throttle valve is adjusted to deliver the desired purge flow rate at that regulated pressure. If the operating pressure is increased, the purge flow rate will remain constant since the pressure at the throttle valve remains constant. Therefore, the efficiency of the dehydrator is not reduced by the increase in operating pressure.
The invention, in one aspect, embodies a membrane gas dehydrator with a housing having an inlet plenum and an exit plenum. An inlet is in communication with the inlet plenum and an outlet in communication with the exit plenum. A bundle of capillary membrane fibers are joined at their ends by common headers and are received in the housing. A fluid stream having water vapor therein flows under pressure P.sub.1 into the tube side of the fibers as a permeate feed stream, while the water vapor permeates through the fibers to the shell side of the fibers forming a permeate-lean feed stream in the fibers and a permeate rich zone on the shell side fibers. A portion of the permeate-lean stream is bypassed from the exit plenum back to the shell side of the fibers to purge the water vapor from the shell side. A pressure regulating valve at pressure P.sub.2 is upstream of a metering valve. P.sub.2 is always lower than P.sub.1 to ensure that if the operating pressure P.sub.1 is increased, the purge flow rate will remain constant.
The invention, in another aspect, comprises a process for dehydrating a fluid stream and includes flowing a fluid stream containing water vapor into membrane capillary fibers under a pressure P.sub.1, flowing the water vapor through the membrane walls to the shell side of the fibers to form a water vapor rich stream and a water vapor lean stream. The water vapor lean stream is discharged while bypassing a portion of the water vapor lean stream and introducing the bypassed portion of the stream as a sweep stream into the shell side of the fibers to absorb the water vapor therein discharging the sweep stream from the shell side of the membrane, and while maintaining the pressure of the sweep stream at a pressure P.sub.2 which is lower than the inlet pressure of the feed stream P.sub.1.